Lou, I couldn't tell much from your pics, but from your description it sounds like the soil [sand] under the liner at the edge gave way and collapsed into the pond -- is that it?
We've recently had a similar situation and the pond is currently "down" as we finish up on the repairs. We have clay and a lot of ground water, so we needed to stabilize the wall edges all around the pond -- lots of hard work for one with only occasional spousal & teen-ager help....! We chose to build a cinder block wall along the 2 sections that had previously collapsed, then a cinder block collar tied those 2 walls together. Won't go into details here, but if you want, I can PM more info.
Actually, the collar part wasn't that hard to do, but did involve a little digging.
But this might be something you can do fairly easily. As one poster has already mentioned use mortar -- I have discovered a technique that uses cement [Portland cement, not concrete mix], sand [mortar is cement + sand], and your native soil. I think it's called "dirt-crete." How creative. But basically what you do is mix your dry soil, sand, & Portland together with a tiller -- don't ask about proportions, though, as I just threw them together. Then use that amended "dirt-crete" mixture to build up your wall, berm, or whatever. Then tamp it down well. I don't have a site that I can refer you to, 'cause most of what I found didn't use it this way, but as walk-ways or backfill.
The addition of the Portland to the native soil acts to stabilize the final product.....we'll see how that works.
So, we have a cinder block collar. The berm will be "dirt-crete," as mentioned above.
